It is basically a trusted database for public keys. You can verify yourself on a number of social networks, through Bitcoin, and your website, etc. Others can then see your key and know you are who you say you are.

By following others, and having people follow you, it creates a web of trust.

Edit - the platform also makes it incredibly easier to encrypt, decrypt, sign, and verify. It is great for people to quickly start using encryption. I know when I first started years ago it was a bit daunting.